Low-density nonmetallic materials with good mechanical properties have found utility in various applications including aerospace applications. For example, radomes that house radar antennas are generally required to have sufficient strength to withstand aerodynamic forces. Nevertheless, the material used to construct the radome cannot interfere with the signals transmitted therethrough. In another example, stiffened sandwich core structures are formed by placing a core of material between spaced-apart face sheets. The core spaces the stiff face sheets apart and stabilizes the face sheets so that the structure has a high flexural rigidity.
In these applications, it is often desired that the materials of construction have as low a density as possible while achieving the required mechanical properties. The low density materials provide many advantages over higher density materials. For example, the low density materials are lighter weight which leads to reduced fuel consumption in aircraft uses. Additionally, low density and weight may also be desirable in specialized non-aircraft applications such as rotating machinery to reduce inertia forces.
Structural metals of relatively low density such as aluminum have been widely used for many decades. More recently, structural nonmetallic materials such as graphite/epoxy composite materials have been developed and have entered service. Foamed polymeric materials have also found many uses. The latter class of materials have densities even less than the polymeric materials from which they are constituted, since a portion of the interior of the material is made up of distributed voids that are produced in the material.
One example of these polymeric foam materials are syntactic foams. Syntactic foams are typically formed of microballoons which are distributed throughout a polymeric matrix. Often, these syntactic foam materials are formed of curable polymeric materials and can include other additives or materials such as reinforcing fibers.
U.S. Pat. No. 4,595,623 to DuPont et al. describes syntactic foams formed from liquid thermosetting resins which can be used to produce lightweight structures for spacecraft applications.
Nevertheless, the use of liquid resin materials in polymeric foam materials has some disadvantages. For example, liquid resin materials when used to form these structures do not produce highly uniform distributions of the solid materials in the foam such as fibers.
Specifically, the fibers will follow fluid flow lines during the formation of the foam structures. As a result, fibers in the syntactic foam will align in the flow direction during mixing and mold filling resulting in an anisotropic foam structure.